Editing Oz (programming language) (section)

===Dataflow variables and declarative concurrency ===
When the program encounters an unbound variable it waits for a value.  For example, below, the thread will wait until both X and Y are bound to a value before showing the value of Z.
<syntaxhighlight lang="erlang">
thread 
   Z = X+Y
   {Browse Z}
end
thread X = 40 end
thread Y = 2 end
</syntaxhighlight>

The value of a dataflow variable cannot be changed once it is bound:
<syntaxhighlight lang="erlang">
X = 1
X = 2 % error
</syntaxhighlight>

Dataflow variables make it easy to create concurrent stream agents:

<syntaxhighlight lang="erlang">
fun {Ints N Max}
   if N == Max then nil
   else 
      {Delay 1000}
      N|{Ints N+1 Max}
   end
end

fun {Sum S Stream}
   case Stream
      of nil then S
      [] H|T then S|{Sum H+S T}
   end
end

local X Y in
   thread X = {Ints 0 1000} end
   thread Y = {Sum 0 X} end
   {Browse Y}
end
</syntaxhighlight>

Because of the way dataflow variables work, it is possible to put threads anywhere in a program and guaranteed that it will have the same result. This makes concurrent programming very easy. Threads are very cheap: it is possible to have 100,000 threads running at once.<ref>{{Cite web |url=http://www.mozart-oz.org/documentation/tutorial/node8.html#chapter.concurrency |title=Archived copy |access-date=29 November 2008 |archive-url=https://web.archive.org/web/20150224185115/http://www.mozart-oz.org/documentation/tutorial/node8.html#chapter.concurrency |archive-date=24 February 2015 |url-status=usurped }}</ref>